Automotive air conditioning compressors require a simple and compact drive means for translating rotation of a drive shaft into axial reciprocation of a number of pistons. One of the most common mechanisms is the so called swash plate, an annular disk that rotates one to one with the shaft, but which is slanted with respect to the shaft, rather than perpendicular thereto. As the shaft rotates, diametrically opposed points near the outer edge of the swash plate shift axially back and forth in opposite directions. This axial motion of the plate can be translated into axial motion of the pistons through a suitable bearing joining the plate to the pistons. Such a bearing mechanism must provide a translational degree of freedom, plus at least two rotational degrees of freedom. Consequently, it is subject to a good deal of friction and continually reversing loading.
The two most common bearing mechanisms use a pair of relatively small balls for each piston, one on each side of the swash plate near its outer edge. The older version of the two ball mechanism uses completely round balls, which turn in small sockets in the piston on one side and which slide on the other side on separate so called "shoes" on the surface of the plate. The shoes are typically bronze or some other friction reducing wear material. An example may be seen in U.S. Pat. No. 4,392,788 to Nakamra et al. Four pieces all together, two balls and two shoes, must be sized and assembled relative to the swash plate thickness to assure that all tolerances are taken up. A newer version of the two ball mechanism eliminates the shoes as a separate piece by flattening off the plate side of the balls and coating them with a wear reducing layer. An example may be seen in U.S. Pat. No. 4,790,727 to Steele. This reduces the piece count by two, but it is substantially more expensive than providing plain, round balls. A substantially different approach is disclosed in U.S. Pat. No. 4,683,803, co assigned, in which a pair of gum drop shaped plastic bearing members, integrating the shoe and ball functions, are loaded under compression between piston sockets and the swash plate. While potentially eliminating the steps of sizing the separate pairs of shoes and balls in favor of a single size, single material plastic piece, two such plastic pieces are still necessary for each piston, and the design has not found its way into production.